Engineering geologicaland geotechnical evaluation of dam spillway (II-C) of bunakha hydroelectric project, Bhutan Himalaya

The 193.3 m long and 64 m wide spillway of Bunakha Hydroelectric Project is proposed 89.50 m from the right abutment to pass the flood discharge and lies mainly on the right abutment and partly in the river section. The spillway area was investigated through detailed engineering geological mapping, exploratory drilling and laboratory testing. The rock mass properties, i.e., joint sets, weathering grade, RQD, RMR, permeability etc. of the rock masses to be encountered during the excavation of spillway have been analyzed in detail. Core samples from the exploratory drill holes drilled at the spillway alignment were tested for physico-mechanical properties of rocks in the laboratory. The mapping details indicated that the major rock types which are exposed to the surface and cover the entire spillway area are foliated gneisses with bands of banded gneisses. On the basis of detailed investigations and laboratory testing, inferences and recommendations have been made which will be helpful during the construction of the project.